Muscle Capillaries Respond Differently to Exercise In Men and Women

While women appear to begin with a lower density of tiny capillaries in their skeletal muscle than do men, this density seems to increase at a higher rate than men as a result of exercise, according to a new analysis by Duke University Medical Center researchers.

Also, after 24 weeks of supervised exercise training, middle-aged overweight adult men and women both saw a similar increase in their exercise capacity, leading the researchers to conclude that men and women's skeletal muscle responds differently to exercise, and that improved skeletal muscle capillary density may play a greater role in women than men for improving exercise capacity.

Capillaries are the tiny blood vessels that carry oxygen-rich blood to the body's tissues, including muscle. Although it is known that skeletal muscle, as well as cardiac muscle, changes as a result of exercise, it is not know whether increased capillary density is equally responsible for the improved exercise endurance between men and women.

"There is sparse scientific data comparing how the muscle capillary density is affected by exercise in men and women," said Brain Duscha, who presented the results of the Duke study today (May 31, 2003) during the 50th annual scientific sessions of the American College of Sports Medicine. "Based on the results of our study, it appears that the skeletal muscle of men and women may adapt differently to exercise and therefore rely on different mechanisms to increase their peak exercise capabilities."

For their study, the Duke researchers enrolled 10 men and 12 women who were sedentary, and who were at risk of developing diabetes or heart disease. Participants exercised for 24 weeks at the equivalent of jogging 20 miles per week. Using thin needles, the researchers took leg muscle biopsies before and after exercise training to estimate capillary density.

Before beginning exercise training, the women had an average baseline capillary density (endothelial cell/muscle fiber) of 1.53, which significantly increased to 1.89 after exercise. Men however, rose from 1.71 to 1.82, an increase not deemed statistically significant.

In addition to analyzing the actual changes in muscle tissue, the researchers also determined the exercise capacity of each participant by measuring the maximum amount of oxygen being consumed by the body during exercise. This calculation is known as the peak VO2 (subscript 2), and it measures the maximum amount of oxygen that can be delivered by circulating blood to tissues in a given period of time.

Both genders increased their VO2 after the exercise program by approximately 20 percent. The men increased from 30.6 ml/kg/min1 to 35.8 ml/kg/min1, while the women increased from 24.5 ml/kg/min1 to 29.1 ml/kg/min1.

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The researchers then performed a statistical analysis that controlled for gender, and they found that capillary density does affect peak VO2 irrespective of gender, Duscha said. The researchers were surprised because men did not significantly increase capillary density, while the women did.

"Interestingly, there was an inverse relationship between the changes in peak VO2 and changes in capillary density for men only," Duscha said. "This suggests men rely on factors other than capillary density to improve exercise capacity. The most interesting finding is that women increased capillary density after exercise training and men did not."

This surprising finding led the researchers to postulate that different factors between the genders led to the equal improvements in peak VO2. One partial explanation, according to Duscha, is that exercise causes the heart's output to improve much sooner after exercise training than the subsequent beneficial effects on skeletal muscle.

"Since men begin with higher capillary density than women, increased levels of oxygen can be delivered to the tissues by acute improvements in cardiac output, thereby increasing the VO2 measurement," he continued. "Since women have lower densities, they do not receive the immediate benefits of improved cardiac output. Instead, women may respond by increasing capillary density in skeletal muscle"

Or, the researchers say, other factors may benefit men, including enzymes that increase the oxidative process or even mitochondria, which are found in all cells and are known as the cell's "power plants." Both use oxygen and would increase the VO2 score. "We hope that additional analysis in our lab with a larger subject population will confirm these findings."

The Duke team was led by cardiologist William Kraus, M.D., who received a $4.3 million grant from the National Heart, Lung and Blood Institute in 1998 to investigate the effects of exercise on sedentary overweight adults at risk for developing heart disease and/or diabetes. The results of that five-year trial, dubbed STRRIDE (Studies of Targeted Risk Reduction Interventions through Defined Exercise), are now being published and presented.